Structure reinforced latex particles

ABSTRACT

A process for preparing a structure reinforced latex particle whose film has improved tensile and elongation properties prepared by an emulsion polymerization comprising a three-stage monomer feed addition. The second monomer feed is an amount from about 5 to about 30 parts per hundred parts monomer in said latex particle and is a polymer of which would have a Tg greater than either the first or third monomer feed. The subject structure reinforced latex particle provides a film having improved tensile properties without a corresponding reduction in elongation. A pressure sensitive adhesive formulated with the structure reinforced latex particle is also provided.

BACKGROUND OF THE INVENTION

The present invention is directed towards the preparation of a latexhaving high tensile strength without a loss in elongation. Thisimprovement in physical properties results in a latex suitable as anadhesive having high shear adhesion without a loss of peel adhesion.

In the adhesive industry there is a great desire for adhesives having abalance of shear and peel adhesion. Unfortunately, shear adhesion iscontributed by the tensile property of the latex film and peel adhesionis contributed by the elongation property of the latex film. Tensile andelongation are antagonistic properties and; therefore, increasing onetends to decrease the other. In order to cope with this problem, variousmethods for preparing latexes have been attempted.

One method, and perhaps most common, is to copolymerize various monomersto obtain the benefit of the individual monomer components. Anotherpolymerization method is to stage the addition of monomers to form astructured latex particle having diverse properties. While either methodhas produced latexes having properties acceptable for adhesives,especially pressure sensitive adhesives, it is still desirable toincrease either shear or peel adhesion without sacrificing the other.

SUMMARY OF THE INVENTION

The present invention is a process for preparing a structure reinforcedlatex particle whose film has improved tensile and elongation propertiesprepared by an emulsion polymerization comprising the steps of:

(a) polymerizing a first monomer feed to form a core region,

(b) polymerizing in the presence of (a) a second monomer feed, a polymerof which would have a glass transition temperature (T_(g)) greater than(a) in an amount from about 5 to about 30 parts per hundred partsmonomer in the latex particle, and

(c) polymerizing in the presence of (a) and (b) a third monomer feed, apolymer of which would have a T_(g) less than (b) to form a shellregion.

The T_(g) of the polymer formed form the (b) monomer feed is generallyat least 25° C. In one aspect, the polymer formed from the monomer feeds(a) and (c) have a similar T_(g) or may, in fact, comprise the samecomposition. A typical monomer feed for (a) and (c) is a mixture ofstyrene and butadiene.

In yet another aspect of the process, the monomer feed (b) can be anamount from about 15 to about 25 parts per hundred parts monomer in thelatex particle. The monomer feed (b) can be styrene or a mixture thereofwhose polymer would have a T_(g) greater than 25° C. The addition of themonomer feeds (a), (b) and (c) can be made in a continuous feed manner.

The present invention also provides for a structure reinforced latexparticle whose film has improved tensile and elongation propertiesprepared by the process described above.

In yet another aspect, the present invention provides for a pressuresensitive adhesive comprising a structure reinforced latex particlehaving improved tensile and elongation properties prepared by anemulsion polymerization process, as described above, and a tackifier.Generally, the tackifier comprises from about 1 to about 90 percent byweight on a solids basis in the pressure sensitive adhesive formulation.

The process of the present invention allows for the preparation of alatex particle having increased tensile properties without aproportional decrease in the elongation properties. Advantageously, thetensile and elongation characteristics of the subject structurereinforced latex particle allows for the preparation of a pressuresensitive adhesive having desirable shear and peel adhesion strengths.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 depicts the stress-strain curve of a structure reinforced latexfilm and a comparative homogeneous latex film.

DETAILED DESCRIPTION OF THE INVENTION

The latexes of the subject invention are prepared by a three-stagepolymerization process which provides good tensile properties without acorresponding reduction in elongation properties. In the area ofadhesives, this characteristic provides excellent shear and peelstrengths. The three-stage latex is also suitable for applications as apressure sensitive adhesive.

Three-stage process is meant to describe a polymerization process wherea first monomer feed having one transition temperature (T_(g)) is fedinto the reactor followed by a second monomer feed having a higher T_(g)and, finally, a third monomer feed, similar to said first monomer feed,is added to the polymerization system. The polymerization process can beeither staged, i.e., each monomer feed individually added andpolymerized prior to addition of the subsequent monomer feed or acontinuous feed process. The continuous feed process involves theincremental addition of each monomer feed such that as one monomer feedis reduced the subsequent monomer feed is increased to provide a smoothtransition between various monomer feeds.

The reinforcing portion of the subject reinforced structured latexparticles is contributed by the second monomer feed in the presentthree-stage polymerization. The second monomer feed, having a T_(g)higher than the first and third monomer feeds, is generally referred toas the hard monomer. Generally, the hard monomer has a T_(g) in excessof that temperature for which the latex is to be employed. Therefore, ina pressure sensitive adhesive (hereinafter, PSA) the hard monomer feedwould have a T_(g) greater than about 25° C.

The second monomer feed, or hard monomer portion of the latex particle,reinforces the first and third monomer portions to provide the novelbalance of tensile and elongation properties. The reinforcement of thefirst and third stage monomers if observed when the second monomer feedcomprises from about 5 to about 30 parts per hundred parts total monomer(hereinafter, phr) of the latex particle. More preferably, the secondmonomer feed or hard monomer comprises from about 10 to about 25 phr.

Typical candidates for the second monomer feed, or hard monomer, arevinyl aromatic monomers whose polymers have a T_(g) greater than about25° C., such as styrene and copolymers thereof. However, homopolymers orcopolymers having a T_(g) higher than the first and third stage monomerfeeds and higher than the temperature at which the latex is to beemployed are suitable. Other suitable hard polymers arestyrene/butadiene copolymers, styrene/isoprene copolymers,methylmethacrylate, copolymers or terpolymers of methylmethacrylate withethylacrylate, butylacrylate, 2-ethylhexylacrylate, sec-butylacrylate orbutadiene, copolymers or terpolymers of acrylonitrile withethylacrylate, butylacrylate or butadiene.

The first and third stage monomer feeds, or soft monomers, are generallyof the same composition; however, variation in monomer content andmonomer type is suitable provided the T_(g) does not exceed that of thesecond monomer feed. The first and third monomer feeds, or softmonomers, have a T_(g) less than said second monomer feed or hardmonomer. Typically, the soft monomer has a T_(g) less than about 25° C.Suitable soft polymers can include styrene/butadiene copolymers having aT_(g) of less than 25° C., copolymers or terpolymers of styrene withisoprene, butylacrylate or 2-ethylhexylacrylate, copolymers orterpolymers of acrylonitrile with butadiene, isoprene, butylacrylate,2-ethylhexylacrylate, sec-butylacrylate, ethylacrylate, butylacrylate,and other soft polymers having a T_(g) less than the second stagemonomer feed.

The first and third monomer feeds comprise the remainder of the latexparticle, i.e., that portion not formed by the reinforcing secondmonomer feed. Generally, the first monomer feed comprises from about 10to about 30 parts per one hundred parts total monomer and the thirdmonomer feed comprises from about 40 to 70 parts per one hundred partstotal monomer of the latex particle.

The polymerization of the three monomer feeds is conducted pursuant toconventional emulsion polymerization techniques. Thus, for example, themonomers to be employed for the particular stage involved are typicallydispersed with agitation sufficient to emulsify the mixture in anaqueous medium which may contain known emulsifying agents (i.e.,surfactants) as well as other ingredients conventionally employed in theart as polymerization aids (e.g., conventional chain transfer agents,etc.). Such monomers are then subjected to polymerization with the aidof a conventional source for generating free radicals, such asconventional free radical polymerization catalyst, activating radiation,etc.

Optionally, conventional seeding procedures can be employed in the firststage polymerization to aid in the control of such polymerization and toachieve the desired average particle size and particle size distributionfor the dispersed first stage copolymer particles. Also, as has beennoted, conventional chain transfer agents can be employed in thepractice of the present invention and, indeed, in polymerization stagesemploying aliphatic conjugated dienes it is preferable to do so.Examples of such conventional chain transfer agents include bromoform,carbon tetrachloride, long chain mercaptans (e.g., dodecylmercaptans,etc.), or other known chain transfer agents. Conventional amounts ingeneral, from about 0.1 to about 10 weight percent based on totalmonomer charge of such chain transfer agents are typically employed insuch preferred embodiments. In addition, it is sometimes desirable tohave small amounts of certain known additives incorporated into thelatex. Typical examples of such additives are surfactants,bacteriocides, neutralents, anti-foamers, etc. Such additives can beincorporated into the latex of the invention in a conventional mannerand at any convenient point in the preparation of such latexes.

The reinforced latexes of the present invention are suitable for use ina variety of applications such as in carpet backsizing, as binders andpaper coating compositions, as binders in other compositions, asadhesives for binding together various types of substrates as freefilms, as film-forming components for protective or decorative coatings,in general paints and the like. The subject reinforced latexes areparticularly desirable for use as pressure sensitive adhesives. Thepressure sensitive adhesives (PSA) are adhesive materials, which whendried, possess a lasting aggressive tack which enables them totenaciously adhere to a wide variety of substrates when applied withonly a light pressure. These adhesives are useful in a variety ofapplications including tapes, labels, wall coverings, floor tiles, andwood veneers.

The subject reinforced latex composition is particularly suitable foruse as a pressure sensitive adhesive in view of the excellent shear andpeel adhesion properties. Generally, when employing the subject latexcomposition as a pressure sensitive adhesive, it is desirable to includea tackifying resin emulsion or tackifier. Tackifying resins useful inthe preparation of pressure sensitive adhesives are commerciallyavailable and are generally disclosed in U.S. Pat. No. 4,189,419, hereinincorporated by reference.

Typically, the resins comprise emulsified resin, partiallydecarboxylated rosin, glycerol ester of polymerized rosin, partiallydimerized rosin, natural resin, hydrogenated wood rosin, plasticizedhydrogenated rosin, aliphatic hydrocarbon resins derived from petroleum,aromatic resins derived from petroleum, terpene resins, coal tar,polyindene resins, ethylene vinylacetate copolymer resins, terpenephenolics, coumarone-indenes, rosin esters, pentaerythritol esters,polydicyclobutadiene resins. The properties of the PSA composition canbe varied for a particular application by the selection of anappropriate tackifying resin.

Naturally, the particular tackifiers employed can contain conventionaladditives, such as softeners, plasticizers, antioxidants and the likewhich can be emulsified along with the tackifying resin or emulsifiedseparately and mixed with the tackifying resin emulsion.

Tackifiers are desirably added in an amount such that the latex is givenadditional tack (quick stick and peel adhesion) without detractingunduly from the shear adhesion. Suitable formulations will typicallyhave from about 1 to about 90 percent, preferably about 20 to about 80percent, and more preferably, from about 30 to about 50 percent byweight on a solids basis tackifier in the formulation.

Typically, the PSA composition can be prepared by blending the desiredamount of reinforced latex and tackifier in any conventional manner. Itis understood that no requirement or limitation to the scope of theinvention is intended as to how the latex and tackifier components arecombined.

The following examples are provided to illustrate specific embodimentsof the present invention.

EXAMPLE I

A three-staged structure reinforced latex was prepared as follows. Allmeasurements are in parts per one hundred parts total monomer unlessotherwise indicated.

The monomeric charges were added to an initial mixture consisting of55.99 parts deionized water, 3.5 parts of a polystyrene seed latex toproduce a latex of about 1000 Å. The initial mixture was agitated andthe temperature was maintained at 90° C. To this initial mixture anaqueous charge was added at time zero and continued until time 270minutes consisting of 200 parts deionized water, 0.5 part of 45 percentsolution of Dowfax®-2A1 surfactant (trademark of The Dow ChemicalComapny for sodium dodecyl diphenyl oxide disulfonate) and 0.7 parts ofsodium persulfate. From time zero to 54 minutes, the first monomer feedconsisting of 5.6 parts styrene, 14 parts butadiene, 0.45 partst-dodecyl mercaptan and 0.6 parts carbon tetrachloride was added.

The second monomer feed consisting of 20 parts styrene was commenced at54 minutes and continued until time 107 minutes.

The third monomer feed was commenced at time 107 minutes and continueduntil time 270 minutes. The third monomer feed consisted of 16.4 partsstyrene, 42 parts butadiene, 0.35 parts t-dodecylmercaptan and 1.8 partscarbon tetrachloride.

In addition, at time 15 minutes and continuing until time 210 minutes, asecond aqueous charge consisting of 20.15 parts deionized water, 0.14parts of a 10 percent solution of sodium hydroxide and 2.0 partsitaconic acid was continuously added to the reaction mixture.

At time 270 minutes all additions were completed and the mixture wascooked down for 60 minutes. The solids were collected, latex filmsprepared and the physical properties measured. The results of thesemeasurements are shown in Table I.

A conventional homogeneous styrene/butadiene latex was also prepared asa comparison. The comparative homogeneous latex was prepared by acontinuous feed process using 70 parts butadiene, 28 parts styrene, 2parts itaconic acid, 2.25 parts t-dodecyl mercaptan and 3 parts carbontetrachloride with all other ingredients and conditions beingessentially equal to the three-stage latex preparation. Naturally, thestructure reinforced latex formulation contains more styrene than thehomogeneous latex due to the intermediate charge of reinforcing styrene.Essentially, the structure reinforced latex as prepared above consistsof a first monomer charge equivalent to 20 percent of the comparativehomogeneous latex monomer feed, followed by 20 percent reinforcingstyrene monomer charge, and finally a third monomer charge equivalent to60 percent of the comparative homogeneous latex monomer feed.

The physical properties for the film prepared from the homogeneous latexare included in Table I as the comparative homogeneous latex.

The adhesive properties of the latexes were measured according toPressure Sensitive Tape Council (PSTC) Procedure Nos. 1, 5, and 7. Thepeel adhesion test involves peeling the tape off at a 180 degree angleafter application under relatively heavy pressure. The quick stick testinvolves peeling the tape off at a 90 degree angle after applicationunder a relatively light pressure. Both tests are reported as the forcerequired to remove the tape. Shear adhesion is measured according PSTCNo. 7 using a 500 gram (g) mass at a 120° F. In this test, a shear force2 degree angle is applied in an oven. The results are reported as thetime required for the bond to fail.

    ______________________________________                                                                     Comparative                                                      Structure    Homogeneous                                      Property        Reinforced Latex                                                                           Latex                                            ______________________________________                                        Tensile (psi)   173          98                                               Elongation (%)  827          865                                              Shear Adhesion (hrs)                                                                          >100         58.2                                             Peel Adhesion (lbs/in)                                                                        1.9          1.9                                              Film                                                                          Stainless Steel 1.6          1.3                                              Quick Stick (lbs/in)                                                                          1.0          .8                                               ______________________________________                                    

The physical data above shows an increase in tensile properties withouta corresponding loss in elongation for the structure reinforced latex.The gain in tensile is demonstrated by the excellent improvement inshear adhesion with no loss in peel adhesion when compared to thehomogeneous latex.

The stress-strain curves for films formed from the structure reinforcedlatex and comparative homogeneous latex of Example I are depicted inFIG. 1. FIG. 1 indicates the improved stress properties of the subjectstructure reinforced latex while maintaining good elongation comparableto the homogeneous latex. Generally, one would expect an increase intensile stress, i.e., to cause an increase in the slope of a linegraphing stress versus elongation.

Table I shows the peel adhesion (a function of elongation) and shearadhesion (a function of tensile) for the structure reinforced latex ofExample I versus the comparative homogeneous latex. The latexes areshown neat and with varying tackifier levels. Pressure Sensitive TapeCouncil Test Nos. 1 and 7 were employed for conducting the peel adhesionand shear adhesion tests, respectively.

                                      TABLE I                                     __________________________________________________________________________    180° Peel Adhesion (lb/in)                                                                      500 g Shear Adhesion (Hrs)                           Latex/          Structured         Structure                                  Tackifier*                                                                          Homogeneous Latex                                                                       Reinforced latex                                                                       Homogeneous Latex                                                                       Reinforced Latex                           __________________________________________________________________________    100/0 1.3       1.6      58.2      >100                                       90/10 0.9       2.3      3.9       >100                                       80/20 1.7       2.1      2.35      37.15                                      70/30 2.1       1.4      1.7       16.85                                      __________________________________________________________________________     *Pentalyn H55WBX ® (Trademark of Hercules, Inc. for a polyethylene        ester of a partially hydrogenated rosin).                                

The data indicate that the structure reinforced latex particlemaintained good peel adhesion while dramatically increasing the tensilerelated property of static sheet.

EXAMPLE II

A three-staged latex, structure reinforced latex particle, was preparedfollowing the procedure outlined in Example I. All measurements are inparts per one hundred parts total monomer unless otherwise indicated.

The first and third monomer feeds, or soft monomer feeds, were identicaland consisted of 56 butadiene, 22 styrene, 2.2 t-dodecyl mercaptan and2.4 carbon tetrachloride. The monomer feed was added from time zero to54 minutes and from 107 to 270 minutes to form the soft regions of thestructured latex particle.

The second monomer feed, or hard monomer feed, consisted of 20 styreneand was added from 54 to 107 minutes to form the hard reinforcing regionof the structured latex particle. Thus, the total monomer feed for thestructure reinforced latex consisted of 20 percent soft monomer followedby 20 percent reinforcing styrene and finally 60 percent soft monomerfeed. All other polymerization conditions were similar to Example I.

For comparison purposes a homogeneous latex was prepared by a continuousfeed process as described in Example 1 but with increased chain transferagent, i.e., 2.75 parts t-dodecyl mercaptan. Each latex was tested neatand with varying tackifier levels as in Example I for peel adhesion andshear adhesion. The values are reported in Table II.

                                      TABLE II                                    __________________________________________________________________________    180° Peel Adhesion (lb/in)                                                                      500 g Shear Adhesion (Hrs)                           Latex/          Structured         Structure                                  Tackifier*                                                                          Homogeneous Latex                                                                       Reinforced latex                                                                       Homogeneous Latex                                                                       Reinforced Latex                           __________________________________________________________________________    100/0 0.9       1.5      1.55      15.65                                      90/10 1.2       1.4      0.67      8.0                                        80/20 3.1       2.2      0.57      7.6                                        70/30 3.5       4.8      0.4       6.9                                        __________________________________________________________________________     *Pentalyn H55WBX ® (Trademark of Hercules, Inc. for a polyethylene        ester of a partially hydrogenated rosin).                                

The values indicate that the structure reinforced latex particle hadmarkedly improved tensile related property of static shear whilemaintaining approximately equal elongation related property of peeladhesion. It is also noted that the increase in chain transfer agentbetween Examples I and II has softened the latex particle which hasaccordingly resulted in lower tensile related values.

EXAMPLE III

A three-staged latex, structure reinforced latex particle, was preparedas in Example II except that the amount of chain transfer agent presentin the first and third monomer feeds, or soft monomer feeds, wasincreased from 2.2 part t-dodecyl mercaptan to 2.6 parts. All otheradditions and monomer feed sequences were similar to Example II.

A comparative homogeneous latex particle was also prepared having theincreased chain transfer agent, i.e., 3.25 parts t-dodecyl mercaptan.Each latex was tested neat and with varying tackifier levels as inExamples I and II for peel adhesion and shear adhesion. The values arereported in Table III.

                                      TABLE III                                   __________________________________________________________________________    180° Peel Adhesion (lb/in)                                                                      500 g Shear Adhesion (Hrs)                           Latex/          Structured         Structure                                  Tackifier*                                                                          Homogeneous Latex                                                                       Reinforced latex                                                                       Homogeneous Latex                                                                       Reinforced Latex                           __________________________________________________________________________    100/0 1.7       1.4      0.08      15.3                                       90/10 1.6       1.3      0.06      3.0                                        80/20 1.9       2.3      0.05      3.1                                        70/30 2.3       4.8      0.03      3.35                                       __________________________________________________________________________     *Pentalyn H55WBX ® (Trademark of Hercules, Inc. for a polyethylene        ester of a partially hydrogenated rosin).                                

The values again indicate as in Examples I and II that the structurereinforced latex has improved tensile related properties of shearadhesion while maintaining the elongation related property of peeladhesion. Also Examples II and III demonstrate the effect of increasingchain transfer agent which has a tendency to soften the latex particlesthus reducing tensile properties.

What is claimed is:
 1. A process for preparing a structure reinforcedlatex particle whose film has improved tensile and elongation propertiesprepared by an emulsion polymerization comprising:(a) polymerizing afirst monomer feed to form a core region; (b) polymerizing in thepresence of said (a) a second monomer feed, a polymer of which wouldhave a glass transition temperature greater than (a), in an amount fromabout 5 to about 30 parts per hundred parts monomer in said latexparticle to form an intermediate layer; and (c) polymerizing in thepresence of (a) and (b) a third monomer feed, a polymer of which wouldhave a T_(g) less than said (b), to form a shell region whereby astructure reinforced latex particle is produced having a soft polymercore, a hard polymer intermediate layer and a soft polymer shell.
 2. Theprocess of claim 1 wherein said addition of monomer feeds (a), (b) and(c) are made in a continuous feed manner.
 3. The process of claim 1where a polymer formed from said (b) would have a T_(g) greater thanabout 25° C.
 4. The process of claim 1 where a polymer formed from saidmonomer feeds (a) and (c) would have the same T_(g).
 5. The process ofclaim 4 where said monomer feeds (a) and (c) are the same composition.6. The process of claim 4 where said T_(g) is less than 25° C.
 7. Theprocess of claim 1 where said monomer feeds (a) and (c) are a mixture ofstyrene and butadiene.
 8. The process of claim 1 where said secondmonomer feed (b) is in an amount from about 15 to about 25 parts perhundred parts monomer in said latex particle.
 9. The process of claim 1where said monomer feed (b) is styrene or a monomer mixture thereofwhose polymer would have a T_(g) greater than 25° C.
 10. A structurereinforced latex particle whose film has improved tensile and elongationproperties prepared by the process of claim 1.